This invention relates to a vibration isolator which is interposed between a vibration source and an object and which reduces or absorbs vibrations transmitted from the vibration source to the object.
A structure having an expandable fluid chamber and used to constitute a vibration isolator for an engine mount or cab mount is known. A structure in which this fluid chamber is sectioned into a plurality of small fluid chambers has also been proposed.
Vibrations are absorbed by virtue of the resistance to a fluid passing through a limitational passage formed in this structure. An example of a means for changing the size of the limitational passage is disclosed in Japanese Utility Model Laid-Open No. 95244/1985. In this example, a valve element is rotated by an electric motor and is thereby moved relative to an opening so as to change the opening area. It is necessary to provide in this device a certain gap between the valve element and a partition wall in order to enable the valve element to smoothly rotate. However, there is a possibility of the fluid leaking through such a gap, making it difficult to attain desired damping characteristics.
Even if an elastic material is used to press the valve element against the partition wall so as to limit the leak of the fluid, the valve element is displaced by the fluid flowing through the passage when the pressure of the fluid is increased. If the pressing force is increased, the resistance to the rotation of the valve element increases, resulting in the need for a large driving force. In addition, such a rotary motion of the valve element restricts the amount of opening of the passage and makes it difficult to set a large opening area.
Japanese Patent Laid-Open No. 55427/1986 discloses an arrangement for opening and closing the opening section. However, in this arrangement, the limitational passage is simply opened or closed, and the size thereof cannot be changed.